1. Field of the Invention
The present invention relates to a method and a device for determining an occupancy of a transmission channel by a radio signal. The invention further relates to a transmitting/receiving device and an integrated circuit with such a device.
The invention falls within the field of data transmission. Although it can be used in principle in any digital communication system, the present invention and its underlying problem will be explained below with reference to a “ZigBee” communication system in accordance with IEEE 802.15.4.
2. Description of the Background Art
Wireless Personal Area Networks (WPANs) can be used for wireless transmission of information over relatively short distances (about 10 m). In contrast to Wireless Local Area Networks (WLANs), WPANS require little or even no infrastructure for data transmission, so that small, simple, power-efficient, and low-cost devices can be implemented for a wide range of applications.
Standard IEEE 802.15.4 specifies low-rate WLANs, which are suitable with raw data rates up to a maximum of 250 kb/s and stationary or mobile devices for applications in industrial monitoring and control, in sensor networks, in automation, and in the field of computer peripherals and for interactive games. In addition to a very simple and cost-effective implementability of the devices, an extremely low power demand of the device is of critical importance for such applications. Thus, an objective of this standard is a battery life of several months to several years.
At the level of the physical layer, in the virtually globally available 2.4 GHz ISM band (industrial, scientific, medical) for raw data rates of fB=250 kb/s, the IEEE standard 802.15.4 specifies a band spread (spreading) with a chip rate of fC=2 Mchip/s and an offset QPSK modulation (quadrature phase shift keying) with a symbol rate of fS=62.5 ksymbol/s.
In an 802.15.4 transmitter for the ISM band, the data stream to be transmitted is first converted to a sequence of PN sequences (pseudo noise) with the use of four databits in each symbol period (TS=1/fS=16 s), in order to select a PN sequence from a sequence set of a total of 16 PN sequences. Each symbol of four databits is assigned in this manner a symbol value-specific PN sequence of 32 PN chips (chip period TC=TS/32 =500 ns=1/fC), which is transmitted instead of the four databits. The sequence set of 16 “quasi-orthogonal” PN sequences, specified in the standard, in this case comprises a first group of eight first PN sequences, which differ from one another only in a cyclic shift of their chip values, and a second group of eight second PN sequences, which also differ from one another only in a cyclic shift of their chip values and from one of the first PN sequences only in an inversion of every second chip value (see IEEE Standard 802.15.4-2003, Chapter 6.5.2.3).
The PN sequences allocated to the successive symbols are linked together and then offset QPSK modulated (quadrature phase shift keying) by modulating, with half-sine pulse shaping, the even-indexed PN chips (0, 2, 4, . . . ) onto the in-phase (I) carrier and the odd-indexed PN chips (1, 3, 5, . . . ) onto the quadrature-phase (Q) carrier. To form an offset, the quadrature-phase chips are delayed by one chip period TC with respect to the in-phase chips (see IEEE Standard 802.15.4-2003, Chapter 6.5.2.4).
The data transmission occurs in principle with the use of frames. Useful data are transmitted in this case in so-called data frames (“data frame,” PPDU), which contain in addition to the actual useful data (“data payload,” MSDU) also check and control data, such as, e.g., a so-called synchronization header. This synchronization header (SHR), present at the start of each frame, comprises a so-called preamble sequence and a “start of frame delimiter” (SFD), which allows the receiver to synchronize and lock onto the data bit stream, so that the transmitted data can then be correctly detected. The preamble sequence has 32 binary zeros, whereas the following SFD field comprises the bit sequence “1 1 0 0 1 0 1” (see IEEE Standard 802.15.4-2003, Chapters 5.4.3 and 6.3.1).
To assure robust data transmission, various security mechanisms are specified at the level of the MAC layer (medium access control). Thus, every frame has a so-called cyclic redundancy code (CRC) at the end, which allows the receiver to detect bit errors. In addition, successful receipt of a data frame is acknowledged by the return of an optional confirmation frame. Furthermore, access to the transmission channel during the so-called contention access periods (CAP) occurs according to the CSMA-CA process (carrier sense multiple access with collision avoidance). In this case, each device that intends to transmit, e.g., a data frame, first waits for a time interval of a randomly selected duration and then checks the channel occupancy. If it is ascertained that the channel is free, the device can subsequently transmit its data. If the channel is occupied, however, the device must wait for another, again randomly selected time interval before it again attempts to access the channel. The use of this CSMA-CA process is intended to avoid collisions if possible, which arise when several devices transmit data simultaneously over the same (frequency) channel. Depending on the network configuration (with or without transmission of a so-called beacon), in this case either a “slotted” CSMA-CA variant directed to a specific time slot or “unslotted CSMA-CA,” not bound to a specific time slot, of this security mechanism is used (see IEEE Standard 802.15.4-2003, Chapters 5.4.1-5.4.4 and 7.5.1).
To ascertain whether the channel may be accessed, the device must be able to determine at any time at the request of the MAC layer whether the channel is occupied (“busy”). For this purpose, it is specified at the level of the physical layer that each device must be capable of performing at least one of three CCA processes (clear channel assessment). In CCA mode 1, a channel is regarded as occupied when an energy above a certain threshold is present. In the modes 2 and 3 of interest here, this is to apply when the signal was detected that has the previously described band spread properties and modulation properties (mode 2) or when such a signal was detected and the energy was above the threshold (mode 3). If the requirement to determine the channel occupancy applies during the receipt of a data frame (“data frame,” PPDU), the channel is to be classified as occupied regardless of the CCA process mode. The receipt of the data frame is regarded here as having been begun after the “start of frame delimiter” (SFD) was detected, i.e., after the receiver is synchronized (see IEEE Standard 802.15.4-2003, Chapter 6.7.9).
Nevertheless, a problematic case is that in which the requirement of determining channel occupancy applies when an 802.15.4 signal has already arrived at the receiver, but the receiver is not yet synchronized. This case can occur particularly shortly after the turning on of the device or waking from sleep mode, when the device wants to send data and determines the channel occupancy in advance. The applicant is not aware of any solution to this problem in the conventional art.
Both coherent and incoherent approaches are known per se to detect data symbols present in an incoming signal. Whereas in coherent approaches the incoming signal is converted into the complex envelope (baseband) with the use of a carrier wave of the same frequency and phase and obtained from the carrier control circuit, this can be omitted in incoherent approaches. Because of the higher realization cost in coherent approaches, which is also accompanied by an increased power requirement, in the present invention an incoherent receiver is used in which the incoming signal is converted at least not in-phase to the complex envelope and the resulting baseband signal is demodulated differentially.